Ion implantation is utilized extensively in the manufacture of microelectronic device products and in other industrial applications. In the ion implantation process, a chemical species is deposited in a substrate by impingement of energetic ions on the substrate. To produce the desired ions, a precursor is required that is susceptible to ionization resulting in an ionized medium that may variously include precursor fragments, positive ions, negative ions, and recombinant ionic and non-ionic species. This ionized medium is processed by extraction, magnetic filtering, acceleration/deceleration, analyzer magnet processing, collimation, scanning and magnetic correction to produce the final ion beam of the desired type of ions that is impinged on the substrate.
Precursors of widely varying type are utilized to form correspondingly varied implanted materials and devices. Illustrative precursors include argon, oxygen, hydrogen, and hydrides and halides of dopant elements such as arsenic, phosphorus, germanium, boron, silicon, etc. Boron in particular is a very widely used dopant element, and in recent years attention has been focused on increasing the efficiency and utilization of existing boron precursors and developing new ones.
One of the main steps in manufacturing of many integrated circuits involves implantation of boron into silicon wafers. Since elemental boron exhibits very low vapor pressure even at high temperatures, utilization of volatile boron-containing precursor compounds is necessary. Currently, boron trifluoride (BF3) is widely used as a precursor for boron implantation. In 2007, worldwide consumption of BF3 for ion implantation was estimated to be on the order of ˜3000 kg, and this volume has continued to grow.
Despite its widespread utilization, BF3 does have disadvantages. The BF3 molecule is very difficult to ionize and only about 15% of all BF3 flowed into the ion source chamber of conventional ionizers can be fragmented. The rest is discarded. Further, only about 30% of the ionized BF3 is converted into B+ ions that can be used for implantation. This results in low B+ beam current that severely limits implantation process throughput.
Some increase of B+ beam current can be achieved by varying the process parameters, such as by raising the extraction current, and by increasing the BF3 flow rate. These measures, however, result in reduced life time of the ion source, high voltage arcing that in turn leads to tool instability, poor vacuum and beam energy contamination.
Throughput limitations associated with low B+ beam current in the use of BF3 have become more important in the semiconductor manufacturing industry in recent years due to the general trend in such industry to utilization of lower implantation energies. At lower implantation energies, the B+ beam experiences a greater blow-out effect due to space charge.
A high-volume manufacturing capability for boron precursors that are reliable and cost-effective in character would therefore provide a major contribution to the art of semiconductor manufacturing as well as other ion implantation applications in which boron doping is employed.